Conventionally, a catalyst supported on a columnar honeycomb structure including a plurality of partition walls that partition and form a plurality of cells penetrating from one bottom surface to the other bottom surface to form flow paths is used for purifying harmful substances such as HC, CO, and NOx contained in an exhaust gas discharged from an engine of a motor vehicle or the like. Thus, when treating the exhaust gas with the catalyst supported on the honeycomb structure, a temperature of the catalyst should be increased to its activation temperature. However, there is a problem that since the catalyst does not reach the activation temperature at starting of the engine, the exhaust gas is not sufficiently purified. In particular, a plug-in hybrid electric vehicle (PHEV) and a hybrid vehicle (HV) sometimes run only with the motor for its traveling, so that they have lower engine starting frequency and lower catalyst temperature at starting of the engine, thereby tending to deteriorate the exhaust gas purification performance immediately after the engine is started.
In order to solve the problem, an electric heating catalyst (EHC) has been proposed. The EHC is configured to be capable of heating the catalyst to the activation temperature before starting of the engine by connecting a pair of terminals to a columnar honeycomb structure made of conductive ceramics and causing the honeycomb structure itself to generate heat by applying electric current. In the EHC, it is desired to provide a uniform temperature distribution by reducing temperature variation within the honeycomb structure in order to obtain a sufficient catalytic effect.
The terminal is generally made of a metal, and the material of the terminal is different from that of the honeycomb structure made of a ceramic. Therefore, for the applications for use at an elevated temperature in an oxidizing atmosphere such as the inside of an exhaust pipe of a motor vehicle, reliability of mechanical and electrical joining between the honeycomb structure and the terminal in an elevated temperature environment is required. Japanese Patent Application Publication No. 2011-246340 A (Patent Document 1) discloses that a pair of electrodes made of a metal layer is provided on a surface of a honeycomb body made of a porous ceramic based on silicon carbide.
The metal layer is comprised of a surface metal layer containing at least Cr and Fe composed essentially of Cr or Fe and a diffusion layer made of a metal silicide formed at a boundary portion with the honeycomb body. The metal layer is diffusion-bonded to the honeycomb body by the diffusion layer made of the metal silicide. Patent Document 1 discloses that a difference of a coefficient of thermal expansion between the honeycomb body and the metal layer can be reduced, so that thermal stress between the honeycomb body and the metal body can be reduced even in an elevated temperature environment and reliability of mechanical joining can be sufficiently ensured.
The honeycomb structure disclosed in Patent Document 1 is produced by heating a honeycomb structure while placing an alloy containing at least Cr and Fe and composed essentially of Cr or Fe on the surface of the honeycomb body. Patent Document 1 discloses that the placing of the alloy on the surface of the honeycomb structure can be performed by, for example, thermal spraying, plating, transfer sheet, printing, dispenser, ink jet, brush coating, vapor deposition, metal foil or the like. It also discloses that the heating in the electrode forming step is preferably carried out in vacuum or in an inert gas, in terms of preventing oxidation of the electrode. Examples of Patent Document 1 disclose that a paste of metal powder composed of each metal electrode material is applied to the surface of the ceramic body, degreased at a temperature of 400° C., and heated at a temperature of 1200° C. for 60 minutes in vacuum.